1. Field of the Invention
The present invention relates to a driving device which is suitable for driving optical members such as lenses to be used in reading devices for recording media, such as camera, DVD, CD and MD, and endoscopes.
2. Description of the Related Art
Conventionally, driving devices, which move movable bodies using expansion and contraction of piezoelectric elements as electromechanical transducers, are known. FIGS. 1 and 2 illustrate an example of a linear-type driving device. A driving device 100 shown in FIG. 1 is constituted so that one end of a piezoelectric element 101 in an expansion-contraction direction is fixed to an end surface of a fixing body 102 by adhesive 102a, and the other end is fixed to a driving shaft 103 as a movable body supporting member by adhesive 101a. The piezoelectric element 101 is connected to a power feeding member 104 by conductive adhesive, and thus a predetermined pulse voltage is applied to the piezoelectric element 101.
As shown in FIG. 2, a movable body 108 is slidable along the driving shaft 103. The movable body 108 is composed of a slider 105, a holding member 106 that holds the driving shaft 103 with respect to the slider 105, and a plate spring 107 which pushes the holding member 106 which holds the driving shaft 103 towards the slider 105. When an optical member such as a lens is attached to the slider 105, the movable body 108 moves on the driving shaft 103 so that the optical member is driven to advance straight.
FIGS. 3(a1) to 3(b) illustrate a driving principle of the driving device 100. When a pulse voltage having a sawtooth waveform having a moderate rising portion (between A and B) and a convulsive trailing portion (between B and C) as shown in FIG. 3(b), for example, to the piezoelectric element 101 of the driving device 100, the piezoelectric element 101 moderately extends to its thickness-wise direction to displace in the moderate rising portion (between A and B) of the pulse voltage as shown in FIG. 3(a2), and the driving shaft 103 fixed to the piezoelectric element 101 moves to a let-out direction. Accordingly, the movable body 108 which is frictionally engaged with the driving shaft 103 moves together with the driving shaft 103.
Thereafter, the piezoelectric element 101 convulsively contracts to its thickness-wise direction to displace in the convulsive trailing portion (between B and C) of the pulse voltage, and also the driving shaft 103 fixed to the piezoelectric element 101 convulsively displaces to a return direction. At this time, as shown in FIG. 3(a3), when an inertial force of the movable body 108 overcomes a frictional force between the movable body 108 and the driving shaft 103 so as to slide, the movable body 108 substantially remains in that position and does not move. As a result, the movable body 108 moves from an initial state shown in FIG. 3(a1) to the let-out direction by a difference in moving amount between expansion and contraction. When such expansion and contraction of the piezoelectric element 101 are repeated, the movable body 108 is driven along the driving shaft 103 to the let-out direction.
On the contrary, the movable body 108 is driven to the return direction according to an opposite principle to the above principle. That is to say, when a pulse voltage of a sawtooth waveform composed of a convulsive rising portion and a moderate trailing portion is applied to the piezoelectric element 101, the piezoelectric element 101 quickly expands to displace in the convulsive rising portion of the pulse voltage, and accordingly also the driving shaft 103 fixed to the piezoelectric element 101 quickly displaces to the let-out direction. At this time, the inertial force of the movable body 108 overcomes the frictional force between the movable body 108 and the driving shaft 103 so as to slide, so that the movable body 108 substantially remains in that position and does not move.
Thereafter, the piezoelectric element 101 moderately contracts to displace in the moderate trailing portion of the pulse voltage, and accordingly also the driving shaft 103 fixed to the piezoelectric element 101 moderately displaces to the return direction. At this time, the movable body 108 as well as the driving shaft 103 displaces to the return direction. As a result, the movable body 108 moves from the initial state to the return direction by a difference in a moving amount between expansion and contraction. When such expansion and contraction of the piezoelectric element 101 are repeated, the movable body 108 is driven along the driving shaft 103 to the return direction.
As such a movable body supporting member (driving shaft) of the driving device 100 which generates a driving force via friction, U.S. Pat. No. 5,589,723 discloses a driving shaft which is manufactured by orienting carbon fiber in an axial direction and hardening it with epoxy resin. In this manufacturing method, however, most of the movable body supporting members have a constant shape in the axial direction, and thus a degree of shape freedom is small.
Further, U.S. Pat. No. 5,994,819 discloses that a hollow shaft made of ceramic is used as the movable body supporting member. Since the movable body supporting member made of ceramic, however, becomes heavier than a movable body supporting member made of a fiber reinforced resin complex, this member is not preferable in that a force from the piezoelectric element 101 is efficiently transmitted. Further, the cost of finishing surface roughness of the movable body supporting member into desired smoothness becomes high.